Alloy with High Core Hardness Suitable for Rapid Nitriding

ABSTRACT

The disclosure is directed to an alloy steel suitable for rapid nitriding, having a steel composition including by weight (%): Carbon: from 0.2 to 0.4; Manganese: from 0.50 to 1.60; Silicon: from 0.50 to 2.0; Chromium: from 0.40 to 1.5; Vanadium: from 0.03 to 0.30; Aluminum: from 0.07 to 0.30; and a balance of iron and other residual elements, where amounts of nickel and molybdenum are limited.

TECHNICAL FIELD

The disclosure relates to generally to a low alloy steel with high core hardness and, more particularly, to a low alloy steel with high core hardness suitable for rapid nitriding.

BACKGROUND

Nitriding is a highly specialized surface hardening treatment that produces a thin but high hardness case on a wide variety of steels. The significant advantages of nitriding over other surface hardening processes are that the case hardness is developed without quenching and that the attendant distortion problems can be minimized. Finishing operations can be eliminated or held to a minimum.

Nitrided surfaces are highly wear resistant and provide anti-galling properties. The nitrided surfaces of steel parts improve the corrosion resistance of the parts. An additional advantage of nitriding is that the surface hardness is resistant to softening by elevated temperatures up to the nitriding temperature.

A nitriding process involves the diffusion of nitrogen into the base steel. A typical nitriding temperature is about 525° C. The surface hardening during the nitriding process does not require quenching. Core properties are not affected by the nitriding process provided the prior tempering temperature is higher than the nitriding temperature.

Although a wide variety of steels can be nitrided, commonly used steels suitable for nitriding include AISI 4140, AISI 4340, and Nitralloy N. The AISI 4140 is most commonly used low alloy steel for nitriding applications, which usually has a core hardness of HRC 28-32 when quenched and subsequently tempered at temperatures higher than the nitriding temperature. The AISI 4340 has more alloy elements compared to the AISI 4140. The AISI 4340 can have a core hardness of HRC 39 and is used for steel parts requiring a high hardenability steel. These AISI 4140 and AISI 4340 type steels contain silicon in amounts of about 0.15-0.35% by weight and nickel in amounts of about 1-2% by weight.

The Nitralloy N, which is a commercially available, is specifically designed for nitriding. The Nitralloy may be quenched and tempered to achieve typical core hardnesses of about HRC 20-25. The advantages of the nitralloy steels are their high response to nitriding and the very high surface hardness equivalent to about HRC 62-65. However, the Nitralloy requires nickel in amounts of 3-5% by weight.

The conventional medium-carbon alloy steels such as AISI 4140, AISI 4340, and Nitralloy type steels are considered as nitriding alloys since they enjoy the benefits of nitriding. However, these alloys require expensive alloy elements such as nickel and molybdenum. There have been efforts to develop medium-carbon alloys steels with reduced amounts of expensive elements. For example, a nitriding alloy steel is disclosed in U.S. Pat. No. 4,853,049 (hereafter “the '049 patent”), entitled “Nitriding Grade Alloy Steel.” The '049 patent is directed to a through hardening nitriding grade alloy steel including aluminum in a range of about 0.07% to 0.30% by weight and vanadium in a range of about 0.03 to 0.20% by weight.

The alloy steel in the '049 patent is similar to AISI 4140 with a typical core hardness of HRC about 28-32 when tempered at 530° C. A higher core hardness could be achieved if the tempering temperature were reduced. However, the nitriding temperature would also need to be reduced to be less than the tempering temperature. Lower nitriding temperatures significantly increase the nitriding cycle time and hence the nitriding process costs. In addition, the hardness gradient in a nitrided part as well as the surface hardness depend heavily on the prior hardness. The alloy steel in the '049 patent and AISI 4140 typically have surface hardnesses of about HRC 56-58 after nitriding. Although the alloy steel disclosed in the '049 patent may not contain large amounts of expensive elements such as nickel and molybdenum, nitriding such an alloy steel takes substantially less time to achieve desired hardened depth that AISI 4140 steel achieves at the comparable nitriding temperature and furnace atmosphere. However, the alloy steel in the '049 patent does not contain sufficient alloying elements to meet the elevated core hardness desired for some highly loaded components.

There is therefore a need for an alloy with reduced amounts of expensive alloy elements, which is suitable for rapid nitriding while providing desired core hardness.

BRIEF SUMMARY OF THE INVENTION

The disclosure is directed to overcoming the problems of conventional nitriding alloy steels. In particular, a through hardening low alloy steel according to the disclosure provides a composition that is economical, adaptable to a variety of quench mediums, maintains high core hardness after tempering and has improved nitriding characteristics. The initial cost of the low alloy steel is reduced due to the reduction of molybdenum, nickel or other strength-improving alloys.

In one aspect, the disclosure is directed to an alloy steel suitable for rapid nitriding, having a steel composition including by weight (%): Carbon: from 0.2 to 0.4; Manganese: from 0.50 to 1.60; Silicon: from 0.50 to 2.0; Chromium: from 0.40 to 1.5; Vanadium: from 0.03 to 0.30; Aluminum: from 0.07 to 0.30; and iron and other residual elements: balance.

In a further aspect, the disclosure is directed to an alloy steel suitable for rapid nitriding, having a steel composition including by weight (%): Carbon: from 0.2 to 0.4; Manganese: from 0.50 to 1.60; Silicon: from 0.50 to 2.0; Chromium: from 0.40 to 1.5; Vanadium: from 0.03 to 0.30; Aluminum: from 0.07 to 0.30; Nickel: 1.0% or less; Molybdenum: 0.1% or less; and iron and other residual elements: balance.

In yet another aspect, the disclosure is directed to a method for manufacturing a steel product made of an alloy steel suitable for rapid nitriding, including: hot deforming the steel alloy to obtain the steel product; heat-treating the hot deformed steel product; tempering the heat-treated steel product; and rapidly nitriding the tempered steel product, where the alloy steel has a steel composition including by weight(%): Carbon: from 0.2 to 0.4; Manganese: from 0.50 to 1.60; Silicon: from 0.50 to 2.0; Chromium: from 0.40 to 1.5; Vanadium: from 0.03 to 0.30; Aluminum: from 0.07 to 0.30; and iron and other residual elements: balance.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 shows a table comparing exemplary alloys according to the disclosure with other conventional alloys.

FIG. 2 is a schematic diagram of an exemplary disclosed process.

FIG. 3 is a schematic diagram of another exemplary disclosed process.

DETAILED DESCRIPTION

A low alloy steel for rapid nitriding treatment is disclosed. The low alloy steel is economically produced without requiring many expensive elements such as molybdenum and nickel. The low alloy steel contains medium-carbons and strong nitride-forming elements such as aluminum. Nitriding is a surface-hardening heat treatment that introduces nitrogen into the surface of the low alloy steel. When the low alloy steel is nitrided, the aluminum forms AlN particles that strain the ferrite lattice, impede dislocation movement, and thereby strengthen the low alloy steel.

According to various implementations of the disclosure, the low alloy steel contains silicon in an amount of 0.5% or higher by weight. A combination of nitride-forming elements and silicon according to the disclosure provides the low alloy steel with desired core hardness without additions of significant amounts of expensive alloy elements such as Ni, Mo and Ti. A low alloy steel according to various implementations of the disclosure may have a chemical composition as listed in Table I:

TABLE I Composition of exemplary low alloy steel in weight present Constituents Concentration by weight (%) Carbon 0.20-0.40 Manganese 0.50-1.60 Silicon 0.50-2.00 Chromium 0.40-1.50 Vanadium 0.03-2.0  Aluminum 0.07-2.0  Iron and other residual elements Balance

In detail, carbon (C) contributes to the attainable hardness level as well as the depth of hardening of steel. For example, a desired amount of carbon in the steel assures resistance to quench cracking and an adequate response to nitriding. In accordance with an aspect of the disclosure, the amount of carbon is at least 0.20% or higher by weight. However, an excessive amount of carbon (e.g., higher than 0.40% by weight in the steel) may cause cracking or distortion in complex-shaped articles and in such cases a less drastic quench medium such as oil may be required. According to various implementations of the disclosure, the amount of carbon is in a range of from 0.20% to 0.40% by weight. In some aspect, the amount of carbon may be in a range of from 0.24% to 0.34% by weight. A steel article formed with an alloy steel according to the disclosure may be quenchable in water, oil, gas or the like, whichever is more convenient.

Manganese (Mn) contributes to deep hardenability and is therefore present in most hardenable alloy steel grades. In accordance with an aspect of the disclosure, the amount of manganese is at least 0.5% or higher by weight to assure adequate core hardness. However, an excessive amount of manganese (e.g., higher than 1.60% by weight in the steel) may cause cracking According to various implementations of the disclosure, the amount of manganese is in a range of from 0.5% to 1.6% by weight. To maintain uniformity of response to heat treatment, a lower amount of manganese of 1.5% or lower by weight may be considered. In some aspect, a narrower range of manganese from 1.00% to 1.30% by weight may be contemplated as well.

Chromium (Cr) contributes to hardenability of steel and is a nitride former thereby enhancing nitride response. To realize these effects, in accordance to an aspect of the disclosure, the amount of chromium is 0.40% or higher by weight. To avoid detrimental nitride response, however, the maximum amount of chromium is limited to 1.5% by weight. In some aspect, a narrower range of chromium from 0.9% to 1.2% by weight may be contemplated as well.

Aluminum (Al) contributes to hardenability and is a good nitrider former. If the amount of aluminum in the steel is too small, not only is there little observable improvement in either hardenability or nitride response but also the benefits are inconsistent. In an aspect of the disclosure, the amount of aluminum is 0.07% or higher by weight. It has also been found that while increasing the amount of aluminum is beneficial to nitrideability, the tendency for case embrittlement also increases. In one aspect, an upper amount of aluminum may be 2.0% by weight. In some aspect, the amount of aluminum in the steel may be 1.0% or less. A smaller amount of aluminum, 0.3% or less by weight, may be contemplated as well.

Vanadium (V) is also an element present in the steel. In an aspect of the disclosure, an amount of at least 0.03% by weight is present to realize a consistently measurable enhancement of case and core hardness. An upper limit of vanadium may be 2% by weight. Vanadium is an expensive element. In accordance to an aspect of the disclosure, the amount of vanadium may be in a range of from 0.03% to 0.30% by weight. A range of from 0.05% to 0.10% by weight may be contemplated to make the best economic use of this element. Alternatively, an amount of vanadium of 0.1% or higher and 0.2% or less by weight may also be contemplated in the presence of a desired amount of silicon in the steel.

Silicon is an element that enhances core hardness of the steel. In an aspect of the disclosure, the amount of silicon is present in an amount of 0.5% or higher by weight. An excessive addition, however, adversely affects not only the toughness and the hardness of the steel, but also other mechanical properties such as cold-forging properties and machinability. Therefore, the limits of silicon are 2.0% or lower by weight. In some aspect, a narrower range of from 0.6% to 2.0% by weight may be contemplated. In various aspects, a range of from 1.0% to 2.0% by weight may be considered as well. It has been found that the unique combination of aluminum, vanadium and silicon within the ranges according to the disclosure greatly contributes to high core hardness and good nitride response.

Nickel (Ni) and molybdenum (Mo) are expensive elements. From the economic standpoint, it would be desirable to reduce the amounts of nickel and molybdenum. According to an aspect of the disclosure, an amount of nickel and/or molybdenum is 1% or less by weight. In some aspect, the total amount of nickel and molybdenum may be 1% or less by weight. In the presence of a desired amount of silicon according to the disclosure, each of nickel and molybdenum may be further reduced 0.1% or less by weight. In various aspects, each of nickel and molybdenum may be reduced 0.01% or less by weight.

Titanium (Ti) and niobium (Nb) are sometimes added to prevent grain coarsening before and after forging. When added with molybdenum and/or vanadium, titanium and niobium form carbonitrides with nitrogen and carbon in the steel, and are effective in enhancing the core hardness and the surface hardness as well. However, an excessively high content of titanium increases carbide-based precipitates to deteriorate the toughness of the steel. Moreover, titanium is an expensive element. According to an aspect of the disclosure, the upper limit of titanium is 0.05% by weight. In some aspect, the amount of titanium may be 0.01% or less by weight. In various aspects, the total amount of titanium and niobium may be 0.01% or less by weight.

Sulphur (S) in small amounts may be beneficial in that it promotes machining However, when the amount of sulphur becomes excessive, deterioration in toughness and corrosion resistance becomes serious. Therefore, according to an aspect of the disclosure, the amount of sulphur may be set to not more than 0.01% by weight. In some aspect, to avoid loss of ductility, the amount of sulphur may be 0.005% or less by weight.

Phosphorus (P) is an element present in the steel as an impurity. A small amount of phosphorus can cause deterioration in toughness or corrosion resistance of the steel. According to an aspect of the disclosure, the amount of phosphorus is 0.03% or less by weight. However it would be desirable that the amount of phosphorus is 0.01% or less.

The remainder of the low alloy steel composition is essentially iron except for nonessential or residual amounts of elements such as impurities which may be present in small amounts within commercially recognized allowable amounts.

FIG. 1 shows a table to compare exemplary alloy compositions according to the disclosure with conventional alloys, AISI 4140, AISI 4340, Nitralloy and '049 patent alloy. In conventional nitrided alloys which contain relatively large amounts of molybdenum and/or nickel, lower contents of alloy elements improve the tempering resistance and reduce sensitivity to temper embrittlement. In contrast, the higher the alloy contents of the nitriding steel, the greater the surface hardness that can be achieved. The compressive residual stress in the nitrided surface layer also increases, which leads to higher strength. However, the higher surface hardness, which is caused by the additional alloy elements, results in a lower tendency for the surface layer to adhere to a wear partner and the increased surface hardness also leads to a greater risk of cracking during mechanical stressing. A low alloy steel according to the disclosure has lower contents of expensive elements compared to the conventional alloys but provide comparably high core hardness with good nitride characteristics.

Manufactured articles, such as shafts, couplings and gears, having a composition according to the disclosure, may be advantageously initially formed to a desired shape by forging or rolling. In one aspect, the formed articles may be hardened by heating to a temperature about 870° C. (1600° F.) for a period of about one hour and then quenched in either water or oil to complete phase transformation. Optionally, the formed articles may be quenched by high pressure gas quenching, which is typically coupled to vacuum heat treating processes. After tempering to precipitate and agglomerate the strengthening particles and thereby provide desired properties, the articles are then nitrided.

As the tempering temperature increases, the amount of chromium and molybdenum carbides increases as well. This reduces the precipitation of nitrides and results in a lower increase in hardness. In accordance to an aspect of the disclosure, by reducing the expensive alloy elements such as molybdenum, nickel and/or titanium, the economic advantages of having lower amounts of expensive alloying elements are achieved, the manufacturing processes can be reduced, and at the same time, by increasing the silicon content, desired core hardness and/or good response to nitriding are achieved as well.

INDUSTRIAL APPLICABILITY

Steels having compositions according to the disclosure may be supplied as pipes, hot-rolled plate, rolled round bars, forgings, round bars, square bars, flat bars, plates and the likes. A low alloy steel according to the disclosure may be obtained by melting, forming and heat treating. FIG. 2 is a schematic diagram of an exemplary manufacturing process 100 according to the disclosure. A low alloy steel according to the disclosure is alloyed 110. In one aspect, depending on the dimensions desired for the final product, a melted steel may be cast.

The steel is hot deformed by forging or hot-rolling 120. For example, for hot forging, the steel may be initially heated to a temperature in a range of about 1100 to 1250° C. The steel may then be hot forged into a desired shape, and control cooled from the forging temperature to achieve a desired microstructure. The forged product may be air cooled using fans or other means of circulating the cooling air.

The hot deformed steel may be quenched and tempered to specific core hardness 130. The tempering may be carried out at a temperature in a range of about from 200° C. to 650° C. The steel product having a steel composition according to the disclosure has good temper resistance.

The tempered steel is machined to form a steel part by rough machining 140 stress reliving 150 and finish machining 160. The tempered steel may be mechanically processed for various applications such as gears, drive shafts, rods, cylinders, spindles, rollers, valves, rings, rails and the likes. Subsequently, the steel part is rapidly nitrided 170. Optionally, the steel part may be lapped or lightly ground as necessary 180. The exemplary manufacturing process 100 may provide the steel part with minimum distortion.

FIG. 3 is a schematic diagram of another exemplary manufacturing process 200 according to the disclosure. In one aspect, after alloying a low alloy steel according to the disclosure 210, the steel is hot deformed by forging or hot-rolling 220 and rough machined 230. The steel is quenched and tempered to specific core hardness 240, and then finish machined to form a steel part 250. Subsequently, the steel part is rapidly nitrided 260. Optionally, the steel part may be lapped or lightly ground as necessary 270. The exemplary manufacturing process 200 may provide the steel with maximum machinability.

Surface treatment may be applied to increase the surface hardness and the wear resistance. Nitriding is a thermo-chemical process by which the surface of a steel part is enriched with nitrogen to form alloy nitrides which improve the wear resistance and form a surface nitride layer which can improve the corrosion resistance of the steel part. For example, nitriding increases surface hardness, wear resistance, resistance to certain types of corrosion, and compressive surface stresses, which improve the fatigue resistance of the steel part. Accordingly, nitrided steel articles are often used for gears, couplings, shafts, and other applications that require resistance to wear due to high stress loading and abrasive environments.

Various methods of nitriding may be employed. One commonly used method of nitriding is gas nitriding. Alternative methods may include salt bath nitriding and plasma nitriding. In gas nitriding the donor is a nitrogen gas, usually anhydrous ammonia (NH₃), which is why it is sometimes known as ammonia nitriding. When ammonia comes into contact with the heated work piece it dissipates into nitrogen and hydrogen. The nitrogen then diffuses onto the surface of the steel creating a nitride layer. The thickness and phase constitution of the resulting nitriding layer can be selected and the process is optimized for the particular properties required.

Nitriding a steel may be carried out in an atmosphere containing partially dissociated ammonia gas at a temperature in a range of 400 to 600° C. In conventional nitriding methods, the process from the commencement to the completion of nitriding usually takes 20-40+ hours. Conversely, in rapid nitriding, it can be seen that nitriding time can be significantly reduced for articles having a steel composition according to the disclosure. For a case depth of 0.3 mm the nitriding time can be reduced on the order of 40% thereby effecting significant cost savings. In one aspect, a time taken from the commencement of nitriding to the completion may be 15 hrs or less. Having reduced amounts of molybdenum, nickel and/or titanium and increased amounts of silicon, a steel composition according to the disclosure is suitable for rapid nitriding and provides the steel with desired core hardness.

Parts made of the steels according to the disclosure may be used for the production of internal combustion engines such as crankshafts, piston pins, cam timing gears, connecting rods and the likes. Additionally, a steel according to the disclosure may be used in a track pin and/or in a track pin joint assembly of a track chain that can be used as part of a tracked undercarriage of a track-type tractor, tracked loader, or any other tracked machine known in the art.

It will be appreciated that the foregoing description provides examples of the disclosed alloy and product. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated.

Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 

We claim:
 1. An alloy steel suitable for rapid nitriding, having a steel composition comprising by weight (%): Carbon: from 0.2 to 0.4; Manganese: from 0.50 to 1.60; Silicon: from 0.50 to 2.0; Chromium: from 0.40 to 1.5; Vanadium: from 0.03 to 0.30; Aluminum: from 0.07 to 0.30; and Iron and other residual elements: balance.
 2. The alloy steel suitable for rapid nitriding according to claim 1, wherein the silicon content in the steel composition is in range of from 1.0 to 2.0% by weight.
 3. The alloy steel suitable for rapid nitriding according to claim 1, wherein the silicon content in the steel composition is in a range of from 1.5 to 2.0% by weight.
 4. The alloy steel suitable for rapid nitriding according to claim 1, wherein the steel composition further comprises by weight: Nickel: 1.0% or less.
 5. The alloy steel suitable for rapid nitriding according to claim 1, wherein the steel composition further comprises by weight: Molybdenum: 0.1% or less.
 6. The alloy steel suitable for rapid nitriding according to claim 1, wherein the steel composition further comprises by weight: a total amount of nickel and molybdenum: 1.0% or less.
 7. The alloy steel suitable for rapid nitriding according to claim 1, wherein the steel composition further comprises by weight: at least one of titanium and niobium: 0.1% or less.
 8. The alloy steel suitable for rapid nitriding according to claim 1, wherein the steel composition consists essentially of by weight (%): Carbon: from 0.2 to 0.4; Manganese: from 0.50 to 1.60; Silicon: from 0.50 to 2.0; Chromium: from 0.40 to 1.5; Vanadium: from 0.03 to 0.30; Aluminum: from 0.07 to 0.30; and Iron and impurities: balance.
 9. A steel product made of the alloy steel suitable for rapid nitriding according to claim 1, wherein the alloy steel is hot-forged at a temperature in a range of about 1100 to 1250° C.
 10. A steel product made of the alloy steel suitable for rapid nitriding according to claim 1, wherein the alloy steel is tempered at a temperature in a range of about from 200° C. to 650° C.
 11. A steel product made of the alloy suitable for rapid nitriding according to claim 1, wherein the steel product is any of pipes, hot-rolled plates, rolled round bars, forgings, round bars, square bars, flat bars, and plates.
 12. A method for manufacturing a steel product made of an alloy steel suitable for rapid nitriding, comprising: hot deforming the steel alloy to obtain the steel product; heat-treating the hot deformed steel product; tempering the heat-treated steel product; and rapidly nitriding the tempered steel product, wherein the alloy steel has a steel composition comprising: by weight(%) Carbon: from 0.2 to 0.4; Manganese: from 0.50 to 1.60; Silicon: from 0.50 to 2.0; Chromium: from 0.40 to 1.5; Vanadium: from 0.03 to 0.30; Aluminum: from 0.07 to 0.30; and Iron and other residual elements: balance.
 13. The method according to claim 12, wherein the silicon content in the steel composition is in range of from 1.0 to 2.0% by weight.
 14. The method according to claim 12, wherein the silicon content in the steel composition is in a range of from 1.5 to 2.0% by weight.
 15. The method according to claim 12, wherein the steel composition further comprises by weight: Nickel: 1.0% or less.
 16. The method according to claim 12, wherein the steel composition further comprises by weight: a total amount of nickel and molybdenum: 1.0% or less.
 17. The method according to claim 12, wherein the steel composition consists essentially of by weight (%): Carbon: from 0.2 to 0.4; Manganese: from 0.50 to 1.60; Silicon: from 0.50 to 2.0; Chromium: from 0.40 to 1.5; Vanadium: from 0.03 to 0.30; Aluminum: from 0.07 to 0.30; and ron and impurities: balance.
 18. The method according to claim 12, wherein the heat-treated steel product is tempered at a temperature in a range of about from 200° C. to 650° C.
 19. The method according to claim 12, wherein the rapidly nitriding further comprises nitriding the steel product at a temperature in a range of 400 to 600° C.
 20. The method according to claim 12, wherein the alloy steel is hot-forged at a temperature in a range of about 1100 to 1250° C. 